Historically for companies involved in "networks" such as ISPs, but the use broadened and became an alternative to .com

.org or .org.au

Suitable for non-profit organisations and community based groups that don't fit any other category. (Also see .asn.au)

.biz

This domain was created to help relieve the pressure on .com domain names. Another option for businesses.

.info

The idea of this domain is that it is intended for informative Internet resources. Some examples include public transport timetables, samples and examples of reference materials, geographical material.

.edu.au

This domain is currently reserved for Australian schools, colleges, universities and educational institutions.

.asn.au

Suitable for incorporated associations, political parties, trade unions, sporting and special interest clubs within Australia.

.id.au

Individuals by real name, pen name, stage name or pseudonym who are Australian citizens or residents.

.tv

Islands of Tuvalu. Because of the popularity of .tv being an abbreviation of
television the government of Tuvalu has a commercial deal that offers worldwide use of .tv for sites offering rich media content.